In a transmission circuit of a radio apparatus operating in Time Division Duplex (abbreviated to TDD) in which data is transmitted and received simultaneously by switching between transmission and reception speedily in a time division manner, a portion of a transmission signal is sampled and a transmission level is checked using a diode detector in order to monitor the transmission signal. FIG. 11 shows an example of the typical configuration of a transmission circuit of a radio apparatus employing the time division duplex (TDD) scheme. Referring to FIG. 11, the transmission circuit comprises a transmission amplifier 1, a directional coupler 2, a transmission/reception changing-over switch (TX/RX SW) 3, a reception circuit (RX) 4, a DC cut capacitor 5, a detection diode 6, an OP amp (operational amplifier) 9, an A/D converter 10, a CPU (Central Processing Unit) 11, and a control logic (control unit) 12.
After a transmission signal is amplified by the transmission amplifier 1 to a desired output level, a portion of the transmission power is extracted out by the directional coupler 2 and a voltage corresponding to a transmission output is detected by the detection diode 6. The output voltage of the detection diode 6 is received by the OP amp 9 (for example, configured as a voltage follower). An output of the OP amp 9 is converted to a digital signal by the A/D converter 10. An output of the A/D converter 10 is supplied to the CPU 11. The CPU 11 monitors the transmission signal and performs alarm-decision of an output level.
For the configuration for detecting (envelope detection) a branch of the transmission power with a diode, see the description in Patent Document 1, for example.
Patent Document 1:
JP Patent Kokai Publication No. JP-P2001-274646A